(1) Field of the Invention
The present invention relates to methods used to fabricate semiconductor devices, and more specifically to a method used to increase the collector - emitter breakdown voltage of a high voltage bipolar device.
(2) Description of Prior Art
The use of BiCMOS, (bipolar--complimentary metal oxide semiconductor), semiconductor chips, used to achieve enhanced performance when compared to semiconductor chips fabricated using only CMOS devices, have again focused attention on bipolar devices. Applications demanding high bipolar breakdown voltages, such as the collector--emitter, or the BVceo parameter, have led to the use of a Buried layer Pinched Collector Bipolar, (BPCB), device, in which the collector--emitter breakdown can be as high as about 30 to 40 volts. Additional increases in BVceo can be achieved by deceasing the concentration of an N well region, the region in which the BPCB device is fabricated in, however at the expense of performance, in terms of the frequency response, (Ft), or the device.
This invention will describe a BPCB, bipolar device, in which polysilicon field plates are placed on field oxide regions, which results in a potential shift, which in turn results in a reduction in surface electric field, thus improving, or increasing, the BVceo parameter to a values greater than 50 volts. In addition the use of the polysilicon field plates, presenting improved breakdown characteristics, thus allows higher N well concentrations to be tolerated, thus allowing higher Ft to be realized. The BPCB bipolar device is easily integrated into a conventional CMOS fabrication process, sharing many process steps, thus reducing the complexity of the BiCMOS integrated fabrication procedure. Prior art, such as Contiero et al, in U.S. Pat. No. 5,589,405, as well as Whitney, in U.S. Pat. No. 5,750,414, describe field plates overlying CMOS devices, however not the configuration, or process sequence, used in this novel application, where the polysilicon field plates are placed on field oxide regions, where the field oxide region is located between a base and collector region, resulting in enhanced breakdown characteristics of a BPCB bipolar device.